Oscillating circuits which produce a variable frequency are well known. One type, inductor-capacitor (LC) oscillators, typically use a variable capacitor to achieve different frequencies, but LC oscillators do not provide consistent frequency output when they are exposed to temperature variations.
Crystal oscillators are far more resistent to temperature changes and have therefore found wide acceptance in the electronics and communications industries. A block diagram of a variable frequency crystal oscillator 10 is illustrated in FIG. 1, and a corresponding diagram of a particular circuit for oscillator 10 is illustrated in FIG. 2A. FIG. 1 shows a phase shift circuit 11 connected in a series loop with a resistor 12, a crystal 13 and an amplifier 14. Crystal 13 is cut to a particular frequency f.sub.r at which it will oscillate with a phase shift of 0.degree.. When a phase shift is introduced into the circuit by phase shift circuit 11, crystal 13 is moved to an equal but opposite phase shift, and this alters the frequency at which the circuit oscillates.
FIG. 2A shows crystal 13 connected in series with resistor 12. The series combination of resistor 12-crystal 13 is driven by a driving transistor 25. Phase shift circuit 11 includes a "varactor" (voltage controlled variable capacitor) diode 20 along with an LC circuit including capacitors 21, 22 and 23 and an inductance 24. The component numbered 16 in FIG. 1 represents an attenuation factor "K" which is a function of capacitors 22 and 23 and transistor 25. Amplifier 14 is represented by a transistor 26. Current supplies 27 and 28 connect the emitters of transistors 25 and 26 to a negative supply voltage V.sub.HH, and the collector of transistor 25 is connected to a positive supply voltage V.sub.CC. The phase imposed by phase shift circuit Il is varied by adjusting V.sub.20, which changes the capacitance of varactor diode 20. Diodes 29 limit the amplitude of the oscillations in the circuit. Varying the capacitance of varactor diode 20 changes the resonant frequency of the LC filter consisting of varactor diode 20, capacitors 21, 22 and 23 and inductance 24 and thereby changes the phase of the signal in oscillator 10.
The frequency of crystal 13 as a function of phase is illustrated in FIG. 3A, which shows that as the phase at which crystal 13 operates is shifted from 0.degree. to +.DELTA.0, the frequency is reduced to f.sub.r -.DELTA.f, and if the phase is shifted to -.DELTA.0, the frequency is increased to f.sub.r +.DELTA.f. An important characteristic of the oscillator 10 is illustrated in FIG. 3B, which shows that the amplitude of the oscillations falls off quite rapidly as crystal 13 is pulled off its preferred frequency by phase shift circuit 11. The function of resistor 12, in fact, is to decrease the Q of crystal 13 and thereby allow a greater frequency shift from f.sub.r. Nonetheless, as the frequency of crystal 13 is pulled downward, at some point the amplitude is reduced to the extent that oscillator 10 ceases to oscillate. As the frequency is pulled upward, at some point oscillator 10 shifts abruptly to a "spurious frequency" f.sub.s, at which it will remain fixed until the circuit is turned off. For example, at a resonant frequency f.sub.r of about 17 MHz, a prior art oscillator can be pulled upward only 1-2 KHz before it will jump to a spurious frequency 2-5 KHz above f.sub.r.
Thus the operative range of oscillator 10 is bounded on the low end by the point at which it simply shuts down, and on the high end by the point at which it jumps to a spurious frequency. In prior art oscillators, this range has been somewhat limited. For example, the prior art voltage-controlled crystal oscillator (VXCO) shown in FIG. 2A, designed to operate at 15 MHz, might have a total deviation of .+-.1.5 KHz before one of these conditions occurs. This range can be approximately doubled by using crystals specially manufactured for use in VXCOs (i.e., to .+-.3 KHz at 15 MHz), but these crystals can be quite expensive.
The principles of this invention are also applicable to ringing circuits, such as those used in television equipment. As is well known, a television transmission signal normally contains a "color burst" in each horizontal blanking interval. The color burst is a signal at a reference frequency (.apprxeq.3.58 MHz in the NTSC standard) which is used in the television receiver's color demodulation circuits. Each color burst lasts approximately 8-10 cycles, and the color bursts are separated (blanked) by about 220 cycles of the reference frequency. FIG. 11A illustrates the color burst portion of a television signal.
A ringing circuit is used to provide a continuous reference frequency in the periods between the color bursts. Each color burst activates the ringing circuit and the latter continues to resonate at a gradually decreasing amplitude until it is activated by the next color burst. The decay which occurs between color bursts must be small enough that the ringing circuit continues to deliver a reference frequency of sufficient amplitude for the color demodulation circuits.
In normal television broadcasts, the reference frequency delivered in the color bursts is quite accurate. However, there can be problems when the signal is received from a video cassette recorder (VCR) or a laser video disk player, since the motors and/or tapes can lead to considerable variation in the reference color (chroma) frequency, particularly if the unit has no time base correction circuitry. VHS video cassette recorders which use the European PAL standard, in particular, exhibit considerable chroma time base instability. A PAL VHS video cassette recorder typically has a chroma phase "jitter" of 15.degree.-25.degree. or more, measured peak-to-peak (.+-.1000 Hz). A standard crystal circuit or color burst ringing circuit has a range of only .apprxeq..+-.200 Hz and therefore merely locks to average out the phase jitter of the color frequency off the tape rather than following the color phase and frequency faithfully. For proper color demodulation in a TV, or for clocking into a memory, the ringing circuit must track the incoming burst more accurately than this.
In the prior art, a wide deviation color frequency oscillator circuit normally includes a lower frequency (580 KHz) LC oscillator and a stable 3.0 MHz VXCO whose output signals are mixed. The upper sideband component from the mixer is selected by a bandpass filter to obtain the 3.58 MHz signal as part of a color burst phase lock loop. Alternatively, a 3.58 MHz LC oscillator circuit may be used with its center frequency stabilized during the vertical blanking interval, as described in U.S. Pat. No. 4,544,943 to Quan, by using a fixed 3.58 MHz crystal oscillator as a reference.
An LC circuit may not be used as a color burst ringing circuit (unless stabilized as described above) because the 8-10 cycle color burst (at 3.58 MHz) is repeated at a 15.7 KHz rate, yielding sidebands separated at 15.7 KHz intervals. If an LC ringing circuit drifts by 15.7 KHz or more, it will lock onto an upper or lower sideband (3.58 MHz .+-.15.7 KHz) rather than the reference frequency. Thus, a highly stable crystal-based circuit is required for this purpose.